1. Field of the Invention
The present invention relates to an antenna module and a radio communication device for use in an RFID (Radio Frequency Identification) system or a near field communication (NFC) system.
2. Description of the Related Art
The RFID system is in widespread use as a billing and charge collection system. In the RFID system, a reader and writer and an RFID tag communicate with each other in a non-contact fashion, and communications are performed between these two devices. Each of the reader and writer and the RFID tag includes a RFID chip to process a signal and an antenna to exchange a radio signal. For example, in an HF-band RFID system, a signal is transmitted and received via an induction field between a coil antenna of the reader and writer and a coil antenna of the tag. Japanese Unexamined Patent Application Publication No. 2004-166175, Japanese Patent No. 3933191, and Japanese Unexamined Patent Application Publication No. 2009-284476 disclose antennas for use in such an HF band RFID system.
Like Felica (registered trademark) introduced during recent years, an HF-band RFID system has been introduced in an information communication terminal, such as a portable telephone, and the terminal itself has been used as a reader and writer or an RFID tag. In such a case, a metal body such as a ground electrode of a printed wiring board or a battery pack may be placed in the vicinity of a coil antenna. However, if a metal body is located close to a coil aperture of the coil antenna, an eddy current is generated to cancel a change in a magnetic field of the coil antenna, and it is thus difficult to achieve sufficient communication distance. For this reason, a coil antenna that remains operative even when it is close to the metal body is needed. Such an antenna as disclosed in Japanese Unexamined Patent Application Publication No. 2004-166175 has a structure in which a magnetic body such as one made of ferrite is sandwiched between the coil antenna and an object on which the coil antenna is to be glued. In such a structure, it is still difficult to sufficiently prevent a magnetic field from entering the metal body, depending on an operation frequency band, thickness and magnetic permeability of the magnetic body, and the like. In other words, the magnetic body needs to be thickened in order to sufficiently control the effect of the magnetic body, and as a result, the size of the antenna increases.
Japanese Patent No. 3933191 discloses an antenna including a magnetic core in a rectangular shape, a first coil arranged at one end of the magnetic core, and a second coil arranged at the other end of the magnetic core. In the antenna, the first coil and the second coil are mutually different in coil-wound directions, and magnetic fields entering between the coils are guided in respective coil directions. The antenna can thus be located close to the metal body. In this antenna, however, communication distances in the top surface direction and end portion direction of the magnetic core are long, but it is difficult to achieve a long communication distance in a direction vertical to the axis of the magnetic core. In other words, there is a direction suffering from low gain.
Japanese Unexamined Patent Application Publication No. 2009-284476 discloses as an antenna excellent in multi-directional directivity an antenna including a magnetic core having a radially extending end portion. A coil is wound around the radially extending magnetic core so that a polarity remains the same. Since the antenna has directivity in a direction in which the magnetic core extends, the communication distance in this direction can be increased. However, the antenna of this structure is complex in the shape of the magnetic core, and it is not easy to wind the coil around the magnetic core.